Initial Points Survey Field Session
AS PART OF the opening of the exhibit Initial Points: Anchors of America’s Grid, Jesse Vogler and Louis Schalk from the Institute of Marking and Measuring (IMAM) led a public field session and surveying exercise in the urban landscape of Los Angeles. The group, a few dozen members of the public who responded to an email announcement, gathered at the CLUI at noon on Saturday, January 28th, for a brief run-through of the exhibit and an overview of the day’s activities.
The group then headed outside the front door of the CLUI, where a five-inch-deep hole had been drilled in the curb a half hour earlier. The base of a brass monument was inserted in the hole, In front of the crowd, and epoxied in place. The brass monument, supplied and inscribed by the City of Los Angeles, will now become part of the city’s official network of more than 17,000 benchmarks, covering a 1,700 square mile area, each indicating their elevation relative to a fixed starting point plane known as the National Geodetic Datum of 1929, based on sea level, as determined originally in 1929.
The group then moved one block west, to the nearest existing benchmark (number 13-02781), embedded in the curb at the corner of Cardiff Street and Venice Boulevard, next to an Indian boutique. While one member of the Institute of Marking and Measuring team held a surveyor's rod (a telescoping graduated pole showing inches and feet, like a tall ruler) over the benchmark, another member set up an optical level on a tripod at a point due west of the new CLUI marker, one block away. With this device, a level elevation plane could be established a few feet above the ground, by looking at the rod through the level. This plane was then transferred down the block (in a number of steps, around objects restricting the view) to the new CLUI benchmark, enabling its elevation to be calculated relative to the existing one.
The Cardiff Street benchmark was at a known elevation of 103.103 feet above sea level. The new benchmark was calculated to be at 104.713 feet above sea level, 1.61 feet higher than the point to the west, suggesting slight downward slope westward, to the ocean a few miles away (note, too, that surveyors use tenths of a foot, not inches, or twelfths).
Once this step was accomplished, the group broke up to re-form at the Baldwin Hills Overlook, a bare hilltop in a nearby park.
This location provides a direct line of sight to the front door of the CLUI, nearly a mile away, and to the new benchmark in the curb. Jesse Vogler of IMAM set up a total station at the top of the mound, and pointed its lens downward. Sarah Simons, of the CLUI, was outside the door holding a rod with a prism attached to it.
The total station’s electronic measuring system emits an infrared signal that bounces off the prism and back to the total station, enabling the device to immediately calculate the distance between them: 4,954.33 feet. With this figure known, the elevation of top of the hill is determined by calculating the angle of the total station’s scope, looking at the prism (the difference between the “optical plane” of the device and the prism), an amount calculated by the device to be 324.06 feet. From this we add the elevation of the benchmark (which we determined earlier to be 104.713 feet above sea level), add the height of the prism above the benchmark (6.00 feet, as indicated on its graduated pole), and subtract the distance of the total station’s focal plane to the ground (measured by the device’s infrared signal to be 4.94 feet), and arrive at the number 429.833 feet: the height of hill we are standing on, above sea level.
Also visible from this point atop the Baldwin Hills Overlook, especially on a clear day such as this, is San Bernardino Peak, a mountain 85 miles to the east, which was used as the original survey point for the federal survey for all of Southern California. Using the peak as the zero angle, the total station determines the CLUI is 222.33.0 degrees relative to it, and using the overlook as a fulcrum, it was thus possible for us to connect the CLUI benchmark to this Initial Point of the public land survey – the rectilinear matrix of the west, and the subject of the exhibit Anchors of America's Grid, in the building 4,954.33 feeet away the grid below. ♦